Rolling explosion-bonded titanium clads



Jufiy 18, 1967 j DE ET AL 3,331,121

ROLLING EXPLOSION-BONDED TITANIUM CLADS Filed D80. 29, 1964 Regions of Direct Titanium Pockets of Homogeneous Alloy Titanium-Zo-Stoel Bonding //Contuining Nerd Phase 2a: l P A I 7 y A A Ja /w L Z /yA/FU 484/0419 /7% 72/74 fl/VMEW P0042 WKO I N VENTOR5 BY I 3,33l,l2l Patented July 18, 1967 3,331,121 ROLLING EXPLUSIUN-BONDED TETANIUM CLADS Joseph L. De Maris, Harrisonville, and Arnold H. Holinrnan, Cherry Hill Township, NJ, and Andrew Pocalyk0, N ewarlr, DeL, assignors to E. l. du Pont de N emours and Company, Wilmington, Deh, a corporation of Delaware Filed Dec. 29, 1964, Ser. No. 421,966 3 Claims. (Ci. 29-475) ABSTRACT OF THE DISCLOSURE This invention relates to a process for rolling certain explosion-bonded titanium-on-steel clads at elevated temperature to yield products having a wavy bond interface with a wave length-to-amplitude ratio of about 20:1 to 100021.

Background of invention Clad metal composites with steel as the backer material are commonly used as a means of combining prime metal properties such as corrosion or oxidation resistance imparted by metals such as stainless steel or Inconel with the economy and strength of steel. Because of its excellent corrosion resistance, but high cost, titanium would be a very desirable metal to clad to steel. Such a clad in the form of plate could be very useful for construction of chemical vessels, boats, and others. In the form of sheet or strip, this clad combination might be utilized for many industrial and consumer applications including kettles, counter tops, skillets, and others.

Various methods have been used in attempting to make titanium-clad steel plate, sheet, or strip; but these methods all suffer from certain deficiencies. For example, during hot roll bonding significant diffusion ensues which results in the embrittlement of the bond interface and the resultant clad has insufiicient bond strength and ductility. Several methods can be used to improve hot roll bonded clads.

One method described in US. 3,125,805 has been to apply a barrier layer, such as vanadium or silver, between the titanium and the steel to prevent the formation of embrittling phases. This method is very expensive because of the cost of the barrier material and the labor involved in putting it on. There is also difficulty in obtaining sufficient uniformity of the barrier layer over the whole area of a large material.

The second method is to use only special types of hacker steels, which, however, restrict the variety of clads possible, are more expensive than many desired backer steels, and do not fully reduce the harmful effects of diffusion.

Brazing is another method used to clad steel with titanium, but this method cannot be used for sizeable pieces because mill power requirements are excessively large.

Explosive means can be used to produce good quality titanium to steel clads as illustrated in US. 3,137,937. However, there are several practical limitations to such clads that can be produced at present by this means. Titanium sheets are not commercially available in widths greater than about four feet. Thus, wider plates which are an advantage in vessel fabrication cannot be made. Large area clads require large amounts of explosive which increases the cost and difficulties of explosive cladding. Very thin backer steels tend to distort and it is difiicult to make titanium-to-steel sheet or strip by explosive means.

Summary of invention and detailed description The process of the present invention solves the problems encountered in the past while making titanium-tosteel clads. A wide variety of backer steels can be used including inexpensive constructional grades and large plates, sheets, or strips can be produced.

The process of this invention comprises rolling a multilayered metal composite at a temperature of about 900 to 1650 F., and preferably 1200 to 1600 F., the composite comprising a layer of titanium joined to a layer of steel by explosive bonding along a wavy interface interspersed with pockets of alloy, which pockets comprise less than about 30%, and preferably less than 15%, of the total bond area and have a composition intermediate between that of the titanium and steel layers, the rolling being conducted within the aforementioned temperature range, until the ratio of the wave length X to the amplitude A of the bonded interface is increased from the range of about 5:1 to 15:1 to the range of about 20:1 to 100021.

The novel products obtained in accordance with this invention are rolled composites comprised of at least two layers, one of titanium and one of steel, the layers being joined along a wavy interface interspersed with pockets of alloy, which pockets comprise less than about 30%, and preferably less than about 15%, of the total bond area and have a composition intermediate between that of the titanium and steel, the interface having a ratio of wave length X to amplitude A of about from 20:1 to 1000:1.

The general nature of the interface and alloy pockets in the starting materials and products of this invention is illustrated in the accompanying drawing. The bond zones in the starting composites, including the alloy pockets therein are substantially ditlusionless, that is, the extent of metallic inter-diffusion at the bond zone adjacent the interface is less than the limit of measurement obtainable with an electron probe and taper sectioning capable of measuring less than 0.2 micron. Although some diffusion does occur during rolling, the products of this invention are far more free from diffusion than conventional diffusion-bonded products. Of course, although the composition is substantially uniform throughout the pockets, i.e., is homogeneous, hard phases are uniformly distributed therein. In general, the pockets contain a hard alloy phase having a diamond pyramid hardness of at least about 400. The titanium and steel layers should have diamond pyramid hardnesses of 450 or less.

Interface as used herein refers to the area along which the titanium and steel layers abut and are bonded. Amplitude is used herein to refer to the average height of peaks measured from the center line of the interface waves, i.e., the average height of the waves therein measured as indicated in the figure. Wave length as used herein refers to the average length of the repeating units in the interface configuration, e.g., the distance between adjacent peaks or valleys, that is, the distance between peaks or valleys measured as indicated in the figure.

as shown in FIGURE 1.

The process of this invention is further illustrated by the following flowsheet:

Explosion-Clad Composite Soaking Rolling -900-l 50F Reheating llii w Rolling Reheating Rolling Descaling V -9 Clad Corrosien-Resistanc'- Plate, Sheet, or Strip In the above diagram, the dashed lines indicate optional process steps described hereinafter. As illustrated above, the composite metal slab is heated to about from 900- 1650 F. and passed through rolls until the desired size reduction is obtained. If large reductions are being accomplished it may be desirable to reheat the composite to within the above range one or more times to reduce the load on the rolls. No separate annealing step is necessary but annealing does take place during reheating of the composite. Descaling, if desired, can be accomplished by mechanical methods such as sand blasting, or other suitable methods. If annealing is carried out, it must be done below about 1650 F.

The unrolled explosively bonded composite used as the starting material should have a bond interface wave-length to amplitude ratio of less than about 15:1, and greater than :1. Ratios outside of this range lead to excessive bond brittleness as the waves are either too small or too large to prevent easy fracture propagation through the bond zone. Heating to above about 1650 F. produces a change in the crystal structure of titanium from closepacked-hexagonal to body-centered-cubic. As a result diffusion is greatly accelerated and hard, and less ductile regions are generated within and adjacent to the bond zone. These phases reduce the quality of the rolled material. Rolling below about 900 F. requires excessive mill power. In addition, smaller reductions are possible at the lower temperatures than about 900 F. due to ease of bond separation as a result of work hardening induced during rolling at these low temperatures.

As indicated hereinbefore, the clad composites used in the process of this invention are prepared by explosive bonding. Preferably, such bonding is done by the low velocity cladding process described in U.S.P. 3,137,937 and incorporated herein by reference. This process cornprises supporting at least one layer of cladding metal parallel to the surface of a layer of metal to be clad, the inside surface of the layer of cladding metal being spaced by a small distance from the surface of said metal to be clad, placing a layer of detonating explosive on the outer surface of the cladding layer or layers, the detonating explosive having a velocity of detonation less than 120%, and preferably less than 100%, of the velocity of sound in that metal in the system having the highest sonic velocity and thereafter initiating the explosive so that the detonation is propagated parallel to the metallic layers. The loading of the detonating explosive, which usually has a detonation velocity of On the order of 1200 to 5500 meters per second, varies with the particular metals being clad and the standoff therebetween, but for titaniumon-steel ranges about from7 to 30 g./in. of commercial low velocity explosives such as grained amatol, /20 AN/ TNT. The standoff on space between the layers varies with the explosive and, generally, increases with the thickness of the layers. In general, for most applications,a

standoff of about 0.05 to 0.75 inch is employed.

The explosive cladding process carried out as described above and in the aforementioned U.S. patent yields composites which can be processed directly as described above in accordance with this invention. The composites need i not be subjected to drastic straightening or other forming operations prior to rolling, although these can be utilized if so desired. In the process of this invention it is necessary to use clads prepared as described above in which the wavy bond zone has discrete, relatively periodic regions or pockets of titanium-to-steel alloy separated by substantially continuous direct titanium-to-steel bonding over more than about 70% of the total area of contact. Composites with such bond zones are most suitable for very drastic reduction in accordance with the process of this invention. Composites with such bond zones are described in more detail in copending application Ser. No. 217,776, filed Aug. 3, 1962, now Patent No. 3,233,312

by George R. Cowan, John I. Douglass and Arnold H.,

Holtzman which. is incorporated herein by reference. In general, such preferred bond zones are favored by using lower detonation velocity explosives and large but not ex-- cessive standoff in the low velocity cladding process described above. FIGURE 1 shows such a bond zone.

The clad composites used in the process of this invention can also be made by using certain preferred conditions of the explosive bonding process described in copending application Ser. No. 264,373, filed Mar. 11, 1963, in the name of Bruno Chudzik which is incorporated herein by reference. This process of the aforementioned application comprises forming a juncture between two metal layers; positioning a layer of a detonating explosive on the external side of one of the metal layers; and initiating the explosive so that at least one of the ratios of the collision velocities to the respective sonic velocities of the metal layers is less than about 1.2. When each of the aforementioned ratios is greater than 1.0, the angle between the metal layers in the collision region should exceed the maximum value of the sum of the deflections produced in the metal layers by oblique shock waves. The preferred conditions which give the undulating bond zones include those where the angle is about from 1 to 5, and the explosive has a detonation velocity of about from 2000 to 4000 m./sec.

Although, for simplicity, the process of this invention is described herewith with respect'to composites having two explosive-clad metal layers, it is also applicable to composites, for example, having three or more such clad layers as well as clads, for example, wherein the steel layer has been joined to another metal, for example, by

explosive or conventional techniques. Normally, titanium layers in the starting clads vary from 0.01 to 0.75 inch while the backer layers vary from 0.5 to inches, although thinner and thicker layers are also suitable.

The titanium useful in the process of the present invention and referred to herein unless otherwise indicated can be unalloyed titanium or can be alloys of titanium wherein the numbers refer to the weight percent of the indicated alloying elements such as Ti.15 Pd, Ti-5 Al-4 FeCr, Ti-6 Al-6 V-2 Sn. Descriptions of these alloys are found in Metals Handbook, vol. 1, 8th ed., p. 1147 et seq, 1961. By unalloyed Ti we refer to grades 1, 2, 3, and 4 as defined by ASTM Designation B265-5 8T.

The steels useful for the present invention and referred to herein unless otherwise indicated include, for example, low carbon steels (less than 0.2% C); medium carbon steels (0.2 to 0.5% C); and high carbon steels (more than 0.5% C); low-alloy steels such as those containing iron and following (numbers refer to percent by Weight) 0.17 C and 0.75 Mn, 0.20 C and 1.25 Mn, 0.22 C and 0.65 Mn, 0.30 C and 1.50 Mn, 1.0 Cr and 1.0 Mo and 0.25 V, 1.0 Cr and 1.0 M0, 0.2 C and 2.25 Ni, 0.30 C and 1.50 Mn and 0.35 Mo; and stainless steels such as 304, 304L, 303, 316, 347, 321, 319, 316L, 410, 430, 446, 201. The preferred grades of steel are those generally employed for construction purposes such as ASTM A212, A285, A204. These steels contain from .15 to .35 carbon.

The amount of reduction obtainable by the process of this invention can be 10:1 or higher. Obviously, any value less than this can also be obtained but, in general, reductions of less than 1.5 :1 have little advantage. The rolling is performed by conventional equipment such as two high, three high, or four high plate mills. Other mills such as tandem or hot planetary mills can also be used. Sandwich rolling can also be performed where two plates, for example, each consisting of composite clad metals can be rolled simultaneously.

The invention is more thoroughly illustrated with the aid of the following examples. Parts and percentages where given are by weight.

Diamond pyramid hardness is an indentation hardness test employing a 136 diamond pyramid indenter and 3,093,521. The composition is readily rolled into sheets and detonates at a velocity of about 4100 meters per second.

A inch-thick layer of ASTM Grade 2 titanium is clad onto a /z-inch-thick plate of ASTM-A212B steel in the following manner. The titanium sheet which measures 3 inches by 6 inches is covered on one side with a layer of the above explosive composition having a weight distribution of 10 grams per square inch. The titanium is placed on the steel plate with a spacing between them of 0.100 inch. The edges of the completed assembly also are sealed with tape, and an electric initiator is attached at one corner of the explosive layer. Excellent bonding of the titanium onto the steel plate results, the wavy interface has a X/A ratio of 6:1. At least one phase of the bond zone has a diamond pyramid hardness of greater than 700. The alloy pockets constitute less than 30% of the total area of the bond zone.

The titanium-steel composite is heated to 1550 F. in 30 minutes and then rolled in 3 quick passes to loss of red heat (about 1000 F.). The final composite undergoes a reduction in thickness of 59.1% from the starting composite and is 0.229 inch thick. When tested by ultrasonic examination the rolled composite is well bonded. It can be bent 180 around a 2/1 D/T bend pin at room temperature without cracking. The final product has an X/A ratio of 36:1 and a bond zone containing less than 15% of alloy pockets.

Example' 2 The explosive employed in this example is a A" thick layer of grained amatol 80/20 AN TN T The titanium is 4" x 7". The separation between plates is 200 mils. An electric blasting cap placed in one corner initiates the explosive which detonates at a velocity of 3600 m./sec. The clad is well bonded and the waves in the bond zone have a X A of 7 to 1 and there is less than 30% of the total area of the bend zone which have alloy pockets.

The clad is then heated to 1550 F. for 30 minutes and rolled to /2 its original thickness without reheat. After rolling the clad is bent in compression 180 around a mandrel twice its thickness without bond separation.

In like manner, the following clads were made and rolled:

X/A X/A Rollin Final Clad Metal Backer Steel (Before (After Reductifin Area Rolling) Rolling) 78 mil thick A TL- thick A2l2B 13 117 3 to l 4 X 21" D0 A thick A204 13 117 3 to 1 4 x 21" D0 thick A387 12 108 3 to 1 4" X 21 D0 1 thick A802 12 192 4 to l 4 x 28 D0 1 thick A212B... 11 891 9 to l 4 x 70 M thick 55A d0 10 90 3110 l 6 x 12 thick 35A T1 1.2 thick A212B 6 54 3 to 1 8 x 14 78 mil 'Ii-O.l5 Pd 1stSh i'ek type 304 13 52 2 to l 6" x 14 variable loads to give one hardness scale for all ranges of hardness from very soft lead to tungsten carbide. The diamond pyramid hardness of the phases in bond zones in the composites of this invention can be determined in the conventional manner by sectioning a sample of composite perpendicular to the layers thereof, etching or polishing the sectioned surface and making a representative number of indentations in the phases observed therein through a microscope.

Example 1 The explosive employed in this example is a thin uniform sheet of a flexible explosive composition comprising 20% of very fine pentaerythritol tetranitrate (PETN), 70% red lead, and, as a binder, 10% of a 50/50 mixture of butyl rubber and a thermoplastic ter-pene resin [mixture of polymers of B-pinene of formula (C H commercially available as Piccolyte S-10 (manufactured by the Pennsylvania Industrial Chemical Corporation). Complete details of this composition and a suitable method for its manufacture are disclosed in US. Patent No.

The clads could be bent in compression without bond separation. Clad G was a large clad whose mechanical properties after rolling were found to be: shear strength, 31,000 psi, yield strength, 51,000, tensile strength, 78,000, and 23% elongation. These mechanical properties demonstrate the high quality of product made by the present invention. In each product the alloy pockets in the bond zone constituted less than about 15 of the total bond area and were substantially homogeneous.

We claim:

1. In the process which comprises rolling titanium/ steel clads at elevated temperatures, the improvements of: (a) providing as the starting material a composite comprising at least one layer of titanium joined to at least one layer of steel by explosive bonding along a wavy interface interspersed with pockets of alloy which pockets constitute less than about 30% of the total bond area and are intermediate in composition between the titanium and steel layers, the ratio of the wave length X to the amplitude A of the bonded interface being about 5:1 to 15:1,

said explosive bonding having been eifected by progressively driving together the surfaces to be bonded at a collision velocity less than 120% of the highest metallic sonic velocity in the system, and (b) rolling said composite at a temperature of about from 900 F. to 1650 F. until said ratio is about from 20:1 to 1000: 1.

2. A process of claim 1 wherein the rolling temperature is about from 1200 to 1600 F. and the ratio X/A is about 20:1 to 1000-:1 corresponding to a reduction in thickness of greater than 1.5: 1.

3. A process of claim 1 wherein the titanium layer is unalloyed titanium and the backing steel contains from about 0.15 to 0.35% C.

7/1961 Kelley 29504 7/1965 Ma et a1. 29-470.1

OTHER REFERENCES Chemical Engineering, May 15, 1961; New Processes for Ti-Clad Steel Plate, byRoy V. Hughson; pp. 194,

196 and 198. 10

JOHN F. CAMPBELL, Primary Examiner. L. J. WESTFALL, P. M. COHEN, Assistant Examiners. 

1. IN THE PROCESS WHICH COMPRISES ROLLING TITANIUM/ STEEL CLADS AT ELEVATED TEMPERATURES, THE IMPROVEMENTS OF: (A) PROVIDING AS THE STARTING MATERIAL A COMPOSITE COMPRISING AT LEAST ONE LAYER OF TITANIUM JOINED TO AT LEAST ONE LAYER OF STEEL BY EXPLOSIVE BONDING ALONG A WAVY INTERFACE INTERSPERSED WITH POCKETS OF ALLOY WHICH POCKETS CONSTITUTE LESS THAN ABOUT 30% OF THE TOTAL BOND AREA AND ARE INTERMEDIATE IN COMPOSITON BETWEEN THE TITANIUM AND STEEL LAYERS, THE RATIO OF THE WAVE LENGTH X TO THE AMPLITUDE A OF THE BONDED INTERFACE BEING ABOUT 5:1 TO 15:1, 